The invention pertains generally to inorganic synthesis and in particular to synthesis of energetic oxidizer salts.
A liquid propellant is referred to as a monopropellant, if the oxidizer is kept with the fuel, or as a bipropellant, if the oxidizer is kept separate until the two are reacted. Presently three liquid oxidizers are used except for special applications such as in propellants for outerspace travel. The first oxidizer, a 90 percent aqueous solution of hydrogen peroxide, has serious stability problems and is therefore difficult to store. Inhibited red fuming nitric acid (IRFNA) is extremely toxic and corrosive. The third oxidizer is 70 percent perchloric acid which has extremely corrosive reaction products and is toxic. These disadvantages of the oxidizers have restricted the use of liquid propellants for rockets, gun systems, and torpedoes.
Aqueous solutions of hydroxylamine salts of perchloric or nitric acid have been shown to be excellent energetic oxidizers for general purpose rockets, gun systems, and torpedoes. The solutions are very energetic, stable, insensitive, and have low freezing points. Many liquid propellant formulations with one of these oxidizers have freezing points below -25.degree. C. The oxidizer solutions themselves have freezing points around -18.degree. C. Hydroxylamine nitrate (HAN) has an additional advantage of producing no corrosive products of combustion; however, this oxidizer is less energetic and stable than hydroxylamine perchlorate (HAP). The major disadvantage and the main reason for the virtual nonuse of these oxidizers, in the past, is their cost.
Recently HAN has been commercially prepared by a batch electrolytic method on a small scale at a reasonable cost. The disadvantages of the method are the large requirements for electricity, batch operation, equipment costs, and scale of operation. This method would be inappropriate for a large production, and it can not produce the more important oxidizer, HAP. Hydroxylamine salts have been prepared, in the laboratory, by an electrolytic method described in CA 17396q 67 (1967). This method would produce a product extremely high in metallic contaminents and is not suitale for large scale production. Also HAP cannot be prepared by this method.
Presently no method exists which can produce HAP at a cost low enough for this oxidizer to be utilized in propellants other than small specialty propellants and no method exists which can produce HAN continuously on a large scale at a low cost. The existing methods have one or more disadvantages, causing the cost of the oxidizers to be too high. Often water is utilized as a solvent, causing serious problems with metallic and other ionic contaminations in the product even after extensive purification. These contaminants can interfere with the performance and stability of the oxidizer. Another common problem is that the product stream is too dilute, therefore requiring expensive distillation or other concentration techniques. On account of the corrosiveness of the reactants, the process equipment for some methods must have glass and glass-lined equipment. Other methods require toxic and/or flammable organic reactants. Often the methods involve many processing steps or long processing times, again increasing the overall cost of the product too much.
The oxidizers are presently prepared by three general methods: aqueous sulfate precipitation, anhydrous precipitation and ion exchange. Each process has one or more features that greatly increase costs.
In the aqueous precipitation process, a saturated aqueous solution of hydroxylamine sulfate is combined with a saturated aqueous solution of sodium, calcium or barium nitrate or perchlorate. These reactions result in the formation of a very difficult-to-filter sulfate precipitate and of an aqueous solution of either hydroxylamine nitrate (HAN) or hydroxylamine perchlorate (HAP). In the case of HAN, the concentration is about 15%. Since most of the useful concentrations of these oxidizers are in the 50 to 85% range water has to be removed by the costly process of distillation. Because the sulfate is difficult to precipitate, residual metal contaminants are present in high concentrations in the final product.
Two methods are disclosed in U.S. Pat. No. 3,420,621 by Watters et al which employ the anhydrous precipitation technique. By this first method, an alcoholic slurry of hydroxylamine sulfate (HAS) is neutralized with alcoholic caustic, resulting in the formation of water as a reaction product. After filtering, the resulting free amine solution is then neutralized with an acid-water mixture. If an aqueous solution of the resulting hydroxylamine salt is desired, the alcohol can be removed by evaporation. In the second method, an alcoholic solution of sodium perchlorate is reacted with an alcoholic solution of hydroxylamine hydrochloride at 50.degree. C. with agitation. After cooling, the sodium chloride precipitate is removed by filtration, leaving a 21% solution of HAP in alcohol. If an aqueous solution of the hydroxylamine salt is desired, water is added and the alcohol is removed by distillation. Hydroxylamine hydrochloride is relatively soluble in methanol but is not available commercially. Hydroxylamine sulfate (HAS) is commercially available but cannot be substituted in this reaction because it is insoluble in alcohols. Both of these syntheses use a flammable and toxic solvent (methanol). Also, a very flammable ether must be added to the alcohol-HAP mixture to precipitate the inorganic impurities.
Two examples of the ion-exchange technique which have been used are disclosed in U.S. Pat. No. 3,508,864 by Thompson et al and in U.S. Pat. No. 3,695,834 by Earl J. Wheelwright. In Thompson et al HAP is prepared by using ion exchange resins to separate interfering ions from the starting hydroxylammonium and perchlorate reactants. In both processes, an excess of acid is used, causing problems of stability and corrosion. Also free perchloric acid can cause a secondary combustion after a torpedo has been fixed. This event presents a serious hazard to personnel in collecting and recharging fired torpedoes. In addition to those problems, organic contaminants from resin degradation are often present in the final product.
Another salt, i.e. sulfate, of hydroxylamine has been prepared by a catalytic reduction of nitric oxide with hydrogen in contact with a platinum catalyst and dilute sulfuric acid. The method is taught in U.S. Pat. No. 3,313,595 by Jockers et al. This synthesis technique has not been successful with the nitrate and perchlorate salts.
Hydroxylamne coordination compounds have been prepared by a homogenuous solution method comprising reacting sodium ethoxide with hydroxylamine hydrochloride in ethyl alcohol. The methods are disclosed in U.S. Pat. No. 3,147,070 by Douglas A. Ranch and in U.S. Pat. No. 3,148,940 by Kenneth O. Groves.
In Applicant's co-pending application for the Synthesis of Hydroxylamine Salts, filed on Jan. 28, 1983, ammonia reacts with hydroxylamine sulfate. The advantage of the synthesis utilizing ammonia is the extremely low cost of the chemical. However, the synthesis has a problem with the by-product, ammonium sulfate, which coats hydroxylamine sulfate and the reaction is just moderately fast.